Refinery and natural gas streams and are typically desulfurized by the Claus process wherein elemental sulfur is produced by reacting hydrogen sulfide and sulfur dioxide in the presence of a catalyst.
The Claus process was discovered over 115 years ago and has been employed by the natural gas and refinery industries to recover elemental sulfur from hydrogen sulfide-containing gas streams for the past 50 years. Briefly, the Claus process for producing elemental sulfur comprises two major sections. The first section is a thermal section where H2S is converted to elemental sulfur at approximately 1,800-2,200° F. No catalyst is present in the thermal section. The second section is a catalytic section where elemental sulfur is produced at temperatures between 400-650° F. over an alumina catalyst. The reaction to produce elemental sulfur is an equilibrium reaction; hence, there are several stages in the Claus process where separations are made in an effort to enhance the overall conversion of H2S to elemental sulfur. Each stage involves heating, reacting, cooling and separation.
In the thermal section of the conventional Claus plant, a stoichiometric amount of air is added to the furnace to oxidize approximately one-third of the H2S to SO2 and also burn all the hydrocarbons and any ammonia (NH3) present in the feed stream. The primary oxidation reaction is shown as follows:2H2S+3O2→2SO2+2H2O  (1)This reaction is highly exothermic and not limited by equilibrium. In the reaction furnace, the unconverted H2S reacts with the SO2 to form elemental sulfur. This reaction is shown as follows:2H2S+SO2⇄3S0+2H2O  (2)Reaction (2) is endothermic and is limited by equilibrium.
In the catalytic section of the Claus process, the unconverted hydrogen sulfide and sulfur dioxide from the thermal stage are converted to sulfur by the Claus reaction (2) over an alumina catalyst. Typically, there are three stages of catalytic conversions. Important features of the Claus reaction in the catalytic stage are that the reaction is equilibrium limited and that the equilibrium to elemental sulfur is favored at lower temperatures.
The Claus process was modified in 1938 by I. G. Fabenindustrie and various schemes of the modified process are utilized today. For feed gas streams containing approximately 40% H2S, the balance carbon dioxide (CO2) and water (H2O), the once through Claus process is generally employed in which all of the acid gas is fed directly to the burner. Three catalytic stages are typically utilized after the initial thermal stage. This scheme will generally produce an overall recovery of 95-97% sulfur. If this recovery efficiency is acceptable, no further processing is required. However, if the recovery efficiency is not high enough (for a variety of reasons and, in particular, environmental constraints) an advanced Claus process such as Comprimo's Super Claus process which has a sulfur efficiency of 99.0% can be utilized. This process consists of the replacement of the final Claus reaction stage by, or the addition of, a reaction stage featuring a proprietary catalyst to promote the direct oxidation of hydrogen sulfide to sulfur selectively in the Claus tail-gas. Air is injected upstream of the reactor. The hydrogen sulfide and oxygen react over the catalyst via the following reaction:2H2S+O2→2S0+2H2O  (3)If a sulfur recovery efficiency of greater than 99% is required, a tail-gas cleanup unit (TGCU) needs to be employed. This type of unit allows for an overall sulfur recovery efficiency of 99.8%. In the United States, a sulfur recovery efficiency of 99.8+% is required for Claus production units generating greater than or equal to 50 STSD of elemental sulfur, hence, a TGCU such as the Shell Scot process is often required. Such processes coupled with a sulfur recovery unit (SRU) can meet and exceed a sulfur recovery efficiency of 99.8+%.
The Shell Claus Off-gas Treating (SCOT) process for removing sulfur components from Claus plant tail gas was first brought on stream in 1973. Since then, the process has been widely used in the oil refining and natural gas industries, with more than 150 units constructed all over the world. In the standard SCOT process, sulfur compounds in Claus plant tail gas are catalytically converted into hydrogen sulfide. After cooling, the hydrogen sulfide is removed by solvent extraction. The SCOT off-gas (the gas not absorbed in the absorber) is incinerated.
The standard SCOT process is able to recover 99.9% of total sulfur, resulting in a 250 ppmv sulfur concentration in the SCOT off-gas. In recent years, the demand for higher sulfur recovery efficiencies has resulted in the development of two improved versions to the SCOT process. These are the Low-sulfur SCOT and the Super-Scot processes. The new processes lower the total sulfur content in the SCOT off-gas to less than 50 ppmv.
An after treatment process which oxidizes all sulfur compounds into SO2 is disclosed in U.S. Pat. No. 3,764,665. This patent disclosed a process for removing sulfur oxides from gas mixtures with a solid acceptor for sulfur oxides wherein the solid acceptor is regenerated with a steam-diluted reducing gas and the regeneration off-gas is fed to a Claus sulfur recovery process. The patent provides for cooling the regeneration off-gas to condense the water vapor contained therein, contacting the cooled off-gas with a sulfur dioxide-selective liquid absorbent, passing the fat liquid absorbent to a buffer zone and then to a stripping zone wherein the absorbed SO2 is recovered from the liquid absorbent and is supplied to the sulfur recovery process. By operating in this manner, fluctuations in the sulfur dioxide concentration of the regeneration off-gas were leveled-out and a relatively concentrated sulfur dioxide stream was supplied to the sulfur recovery process at a substantially constant rate. Although this process supplies relatively concentrated sulfur dioxide to the sulfur recovery process at a substantially constant rate, the off-gas must be cooled and the fat liquid absorbent must be transferred to a buffer zone before the absorbed SO2 can be stripped. Therefore, what is needed is a simpler process whereby these steps are eliminated and energy costs reduced.
In the acceptance apparatus as described in U.S. Pat. No. 3,764,665, solid acceptors are used which are able to accept sulfur oxides and release them again in the form of sulfur dioxide on being regenerated. To this end, carbon-containing adsorbents are disclosed as useful. In this case the sulfur oxides are retained as sulfuric acid in the pores of the carbon adsorbent. After saturation of the adsorbent with sulfuric acid, the carbon-containing adsorbent can be thermally regenerated at 400° C. with the exclusion of oxygen. This yields a sulfur dioxide rich regeneration of off-gas which also contains carbon dioxide, nitrogen, and water vapor. The removal of sulfur compounds in the form of sulfur oxides, under oxidative conditions, i.e., in the presence of oxygen, is preferably affected at temperatures from 325° C. to 475° C. Regeneration under reductive conditions takes place in the same temperature range. Preferably, acceptance and regeneration are affected within this range at the same or virtually the same temperature. At the temperature of adsorption as disclosed in this patent, it is likely the carbon adsorbent is acting as a reducing agent and being consumed as CO2, which is formed during regeneration of the adsorbent. Accordingly, continual replacement of the carbon adsorbent will be necessary.
U.S. Pat. No. 5,514,351 discloses a process of recovering sulfur from a Claus tail gas by forming a sulfur oxide enriched gas stream and contacting the sulfur oxide enriched gas stream with a solid adsorbent bed to extract the sulfur oxides and retain them as sulfur compounds, thus forming a sulfur oxide depleted stream. The sulfur compounds are retained in the bed in the form of inorganic sulfates, sulfur oxides or combinations thereof. The adsorbent bed is then contacted with a reducing gas stream to reduce the retained sulfur compounds to hydrogen sulfide and/or sulfur dioxide and thereby form a hydrogen sulfide and/or sulfur dioxide bearing stream. Sulfur is recovered from the hydrogen sulfide and/or sulfur dioxide bearing stream, and the sulfur oxide depleted stream may be sent to an incinerator or vented through a stack. While in the adsorbent mode, the adsorbent bed is at an elevated temperature of from about 900° F.-1,400° F. Similar to the previous patented process described immediately above, high temperature adsorption causes useful adsorbents such as carbon to react with and be consumed by the sulfur oxides, requiring significant and frequent replacement of the adsorbent.
The objective of this invention is to provide a lower cost solution to the recovery of sulfur from a Claus unit tail gas stream than possible using existing technology and the processes described in the above prior art patents. The current market leading solution for the recovery of sulfur from Claus tail gas streams is still the Shell SCOT process. Unfortunately, the Shell SCOT process costs approximately ½ to ⅓ the cost of the Claus plant itself. Accordingly, lower cost alternatives to the Shell SCOT unit to recover the last 5% of the sulfur leaving the Claus plant in the exhaust gas stream would be welcomed.